Because audio equipment installed in vehicles are essentially expensive, light weight and small in size, audio equipment has historically been the subject of much theft. With the thriving aftermarket network, the stolen equipment may be installed in almost any other vehicle.
Automobile manufacturers have employed numerous methods to prevent the audio equipment from being stolen out of the vehicles. For ease of description, the audio equipment will be referred to as a car radio. It is, however, understood that the audio equipment would encompass any sound system installed within an automobile, including cassette tapes and compact disc ("CD") players. One such method employed by automobile manufacturers to reduce car radio theft is to use special security screws to fasten the radio to the vehicle. These screws may only be unfastened using specially adapted tools.
Another method used to prevent audio equipment theft is to equip the car radio with a security code and a flashing anti-theft LED. Such a system requires the operator to manually input a multi-digit security code to "unlock" the car radio whenever the radio has been disconnected from the vehicle battery. A variation of this anti-theft method is to allow for after-market initialization of the car radio. Here, the consumer inputs a security code into the car radio's memory during initialization. Once the car radio is disconnected from its power source, for example, the car battery, the radio locks up. The operator must then reinsert the same security code to unlock the car radio. For example, U.S. Pat. No. 4,720,700 to Seibold et al., U.S. Pat. No. 4,743,894 to Bochmann, and U.S. Pat. No. 4,683,462 to Takeda et al. all disclose car radio security systems that automatically sense whenever the car radio is disconnected from its power source. Whenever the operating voltage is interrupted, the car radio shuts off until a code word, known only to the authorized user, is reentered by the authorized user. Similarly, U.S. Pat. No. 4,808,981 to Seibold discloses an automotive electronic communication apparatus which prevents the connection of an external battery to a car radio prior to stealing it, in order to prevent an interruption in the power supplied to the car radio. Here, the system measures the impedance of a connected accessory, for example the loudspeaker, and if the impedance of the loudspeaker does not match the impedance stored in the memory of the car radio, the radio will not operate, until the appropriate code word is reinserted.
Yet another method used to prevent car radio theft is to equip the car radio with a removable face plate. Whenever the operator leaves the vehicle, the operator removes the radio face plate, thereby rendering the car radio inoperative. Along these same lines, removable radios have also been designed. Here, the customer removes the entire radio when leaving the vehicle.
Each of these prior art methods are either cumbersome or inconvenient to the consumer. For the methods where the radio, or parts of the radio, are removed from the vehicle, the consumer is required to carry the radio around with him/her. For the methods employing a security code, the consumer is required to memorize this security code in order to "unlock" a radio which has been removed from a permanent power source. Also, the security code would have to be reentered every time the battery is changed or disconnected for ordinary maintenance of the vehicle.